This invention relates in general to vehicle control systems and in particular to a method to detect shorted solenoid coils while the coils are excited by a high frequency Pulse Width Modulated voltage.
Motor vehicles are becoming increasingly sophisticated, especially with regard to electronic controllers for various onboard systems. The electrical to mechanical interfaces for many of these systems include a coil that is used to displace a mechanical device, such as a valve armature with the displacement of the armature being a function of the current flowing through the valve. Several examples include solenoid valves that control vehicle brakes in Anti-Lock Brake, Traction Control and Vehicle Stability Control Systems, solenoid valves that control torsion rods in Active Suspensions Systems and coils that control steering assist in Electro-Hydraulic Steering Systems.
Typical control systems utilize an electronic switch, such as a Field Effect Transistor (FET) to control the current flowing through the coil of a solenoid valve. The FET would have a drain terminal connected to one end of the solenoid coil and a source terminal connected to ground. The other end of the solenoid coil would be connected to a power supply, while the gate of the FET would be connected to a control port of an Electronic Control Unit (ECU). Typically, the ECU control port will be either “low” at ground potential or “high” at a fixed voltage, such as five volts. When the control port is low, the FET is in a non-conducting state and blocks current flow through the solenoid coil while, when the control port is high, the FET is in a conducting state, allowing a current to flow through the solenoid coil.
Should the solenoid coil become shorted, a large current would flow through the FET when it is in a conducting state, which may damage the FET. Accordingly, it is known to test the continuity of the solenoid coils for shorts upon initial startup of a vehicle. However, it would be desirable if the coils also could be tested while the vehicle and the control system are being operated. This would be especially useful for instances when the FET gate is excited by a pulse width modulated gate voltage having a variable duty cycle. In such a case, the times that the FET is on or off vary as the duty cycle changes. Accordingly, it would be desirable to have a testing method for coils that could be implemented while the system is being controlled with a variable duty cycle FET gate voltage.